Measuring the ratio of the gas and dust emission radii of protoplanetary disks in the Lupus star-forming region

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
e-mail: esanchis@eso.org
² Universitäts-Sternwarte, Ludwig-Maximilians-Universität München, Scheinerstrasse 1, 81679 München, Germany
³ Excellence Cluster Origins, Boltzmannstrasse 2, 85748 Garching bei München, Germany
⁴ INAF/Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁵ School of Cosmic Physics, Dublin Institute for Advanced Studies, 31 Fitzwilliams Place, Dublin 2, Ireland
⁶ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁷ Joint ALMA Observatory, Av. Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁸ Núcleo Milenio Formación Planetaria - NPF, Universidad de Valparaíso, Av. Gran Bretaña 1111, Valparaíso, Chile
⁹ European Southern Observatory, 3107, Alonso de Córdova, Santiago de Chile, Chile
¹⁰ Department of Astronomy, Cornell University, Ithaca, NY 14853, USA
¹¹ Atacama Large Millimeter/Submillimeter Array, Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura 763-0355, Santiago, Chile
¹² CENTRA, Faculdade de Ciências, Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal
¹³ Lunar and Planetary Laboratory, The University of Arizona, Tucson, AZ 85721, USA
¹⁴ Earths in Other Solar Systems Team, NASA Nexus for Exoplanet System Science, USA
¹⁵ Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA

Received: 21 October 2020
Accepted: 18 January 2021

Abstract

We performed a comprehensive demographic study of the CO extent relative to dust of the disk population in the Lupus clouds in order to find indications of dust evolution and possible correlations with other disk properties. We increased the number of disks of the region with measured R_CO and R_dust from observations with the Atacama Large Millimeter/submillimeter Array to 42, based on the gas emission in the ¹²CO J = 2−1 rotational transition and large dust grains emission at ~0.89 mm. The CO integrated emission map is modeled with an elliptical Gaussian or Nuker function, depending on the quantified residuals; the continuum is fit to a Nuker profile from interferometric modeling. The CO and dust sizes, namely the radii enclosing a certain fraction of the respective total flux (e.g., R_68%), are inferred from the modeling. The CO emission is more extended than the dust continuum, with a R_68%^CO/R_68%^dust median value of 2.5, for the entire population and for a subsample with high completeness. Six disks, around 15% of the Lupus disk population, have a size ratio above 4. Based on thermo-chemical modeling, this value can only be explained if the disk has undergone grain growth and radial drift. These disks do not have unusual properties, and their properties spread across the population’s ranges of stellar mass (M_⋆), disk mass (M_disk), CO and dust sizes (R_CO, R_dust), and mass accretion of the entire population. We searched for correlations between the size ratio and M_⋆, M_disk, R_CO, and R_dust: only a weak monotonic anticorrelation with the R_dust is found, which would imply that dust evolution is more prominent in more compact dusty disks. The lack of strong correlations is remarkable: the sample covers a wide range of stellar and disk properties, and the majority of the disks have very similar size ratios. This result suggests that the bulk of the disk population may behave alike and be in a similar evolutionary stage, independent of the stellar and disk properties. These results should be further investigated, since the optical depth difference between CO and dust continuum might play a major role in the observed size ratios of the population. Lastly, we find a monotonic correlation between the CO flux and the CO size. The results for the majority of the disks are consistent with optically thick emission and an average CO temperature of around 30 K; however, the exact value of the temperature is difficult to constrain.